Illumination devices comprising luminescent material for converting LED light are known in the art. Transmissive ceramic layers or luminescent ceramics, which can be used for such applications, and the method of preparation thereof, are also known in the art. Reference is for instance made to US 2005/0269582, which discloses a semiconductor light emitting device combined with a ceramic layer which is disposed in a path of light emitted by a light emitting layer. The ceramic layer is composed of or includes a wavelength-converting material such as a luminescent material.
U.S. Pat. No. 7,196,354 discloses another type of device, which comprises a light-generating region adapted to generate light, a wavelength-converting region, and a thermally conductive region. The wavelength-converting region is capable of absorbing light generated by the light-generating region and emitting light having a different wavelength than a wavelength of the light generated by the light-generating region. The thermally conductive region is in contact with the wavelength-converting region and is capable of conducting heat generated in the wavelength-converting region. The thermally conductive region comprises a material having a thermal conductivity greater than a thermal conductivity of the wavelength-converting region. The light-emitting device is adapted to emit light having a total power greater than 0.5 Watts. The thermally conductive region is in contact with a heat sink.